Otto cycle internal combustion engines have long been a source of exhaust-gas emissions which are considered to be deleterious in the atmosphere. Accordingly, various governmental agencies have imposed severe limitations on the amount of pollutants, such as nitrogen oxides and carbon monoxide, which may be emitted by such engines. In particular, large displacement gaseous fuel engines are subject to stringent governmental control due, in part, to the type of fuel which is often ingested by such power plants.
Many industries use stationary engines of large displacement to operate pumps, generators, compressors and so forth. For example, gaseous-fueled engines are commonly found in sewage treatment plants and comprise large, stationary Otto cycle internal combustion engines which are fueled by digester gases and used to operate pumps for sewage and the like.
For example, organic solids reduction process may involve the anaerobic digestion of solid waste and water, or sewage sludge slurry, over a number of days to produce a methane-rich gas. Bioreactor gas is prepared from solid waste by shredding and air classification, followed by blending with water to produce a mixture of 10-20% solids concentration. Digester gas is produced from a slurry which is heated and placed in a mixed digester at about 33.degree. C. for ten to fifteen days, and the digester gas is withdrawn from this mixture. The bioreactor gases primarily contain methane, carbon dioxide and ammonia. Digester gas contains methane, carbon dioxide and traces of other gases. These gases are mixed with air prior to combustion, and form significant amounts of nitrogen oxide and carbon monoxide which are subject to stringent emissions control.
The basic principle of the invention is to provide an apparatus and method for the injection of air into the intake port, directly upstream from the intake valve, to form a quantity of air or lean air/fuel mixture at the intake valve in a manner such that when the valve is subsequently opened, the quantity of air partially fills the cylinder during the suction stroke to define a portion of an incombustible lean mixture adjacent the piston with the remaining portion of the cylinder adjacent the spark plug being filled with a gradually more concentrated (i.e., combustible) fuel-air mixture. This method of air injection has surprisingly been found to significantly reduce carbon monoxide emissions in digester gas engines, particularly at lower engine speeds, i.e., 200 to 260 RPM. In addition, the air-fuel ratio setting is modified to reduce nitrogen oxide formation and the engine spark timing is varied over the operating range of the engine which further reduces nitrogen oxygen formation. This relationship is determined by calibrating the engine. The information obtained with respect to the spark timing is used to prepare a preprogrammed timing device to produce the desired spark angle for any given engine speed. A particularly advantageous spark advance curve is programmed to include a spark advance which increases monotonically from about 15.2 to 16 degrees at 200 RPM, about 17.0 to 17.2 degrees at 240 RPM, about 18.4 to 19 degrees at 280 RPM, and to about 19 to 19.5 degrees at 300 RPM. By this apparatus and method, the engine exhaust gas for a digester gas engine is in full compliance with the most stringent regulations for nitrogen oxide and carbon monoxide emissions.
More particularly, an apparatus and method are provided for controlling engine exhaust emissions in the exhaust gas of a spark-ignition Otto cycle internal combustion engine which includes permitting air to be injected into the intake port of the engine immediately prior to the intake valve in a manner by which a substantial portion of the intake manifold adjacent to and upstream from the intake valve is filled with air while the intake valve is closed, and this air is drawn into the combustion chamber by the intake stroke of the associated piston so that the gas which ultimately fills the chamber has a gradient of fuel concentration with a portion nearest the piston consisting essentially of air and the portion nearest the spark-ignition means consisting essentially of the fuel-air mixture which is drawn from the intake manifold upstream from the air-containing portion. The method further includes initially operating the engine without air injection and retarding the spark timing means from optimal tuning angle; regulating the air-fuel mixing means and thus the ratio of the air-fuel mixture which is drawn into the combustion chamber to achieve an oxygen level in the exhaust gas of from about 1.5 to about 1.7% by volume; opening the inlet air control means and admitting air to the intake port to lower the emissions in the exhaust gas without lowering the engine speed; setting the engine to operate at a given speed within the operating range, adjusting the spark timing means and recording the spark angle necessary to obtain the minimum emissions for a given RPM; repeating the setting, adjusting and recording steps over a plurality of engine operating speeds and determining a spark timing curve for minimum emissions over the operating range of the engine; and using the determined spark timing curve to program the spark timing means so that minimum nitrogen oxide emissions over the operating range of the engine is obtained.
Broadly, the invention comprises creating a gradient of fuel concentration in the cylinder (i.e., a stratified charge having an infinite number of strata), the charge having an incombustible fuel-lean portion next to the piston and a relatively fuel-rich portion next to the spark plug in the combustion chamber, and preprogramming the spark timing means to produce a spark advance to further minimize the emissions for the operating speed of an Otto cycle engine. The invention is particularly advantageous when employed with a gaseous fuel engine such as a digester gas engine.